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Streptococcus infections

Streptococcus sp. infections play an important role in equine medicine all over the world, with the ability to cause severe disease of varying and often fatal symptoms in horses.

There are two key pathogenic bacterial species of Streptococcus sp. in horses:

  • Streptococcus equi ssp. equi
  • Streptococcus equi ssp. zooepidemicus

Strangles results from an infection of Streptococcus equi, causing upper respiratory tract inflammation, a muco-purulent nasal discharge, and inflammation of the regional lymph nodes. This infection results very quickly in the formation of an abscess (usually in the sub-mandibular area), which may be small and resolve over time, or may become a very large and open abscess, requiring appropriate treatment. Strangles is highly contagious, and horses can be carriers and shed the infection, although they themselves may show no outward clinical signs. This means that without proper preventative husbandry and a vaccination strategy, an infection could result in large outbreaks of this disease in horse populations. S. zooepidemicus is also known to cause paralysis in foals (polyarthritis) and sporadic abortions in pregnant mares.

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  • Symptoms may vary according to different Streptococcus species:

    Infection with Streptococcus equi ssp. equi:

    Strangles (Lymphadenitis equorum):

    A contagious infection, characterized by the occurrence of abscesses in lymph nodes draining the upper respiratory mucous membranes

    • Incubation period: 3-6 days
    • First sign is often the refusal of eating and drinking
    • Fever up to 41°C
    • Catharrhal inflammation of the upper respiratory mucosa and swelling of lymph nodes of the pharynx and mandibular region, which become abscessed
    • Mucopurolent nasal discharge
    • Infection of the central nervous system or heart, in very severe cases
    • Course of disease approximately 2 weeks for individual, but an outbreak in a herd may extend over several months
    • Chronic form- known as “bastard strangles” may occur, with subsequent infection of other organs beyond the upper respiratory system.

    Infections with Streptococcus equi ssp. zooepidemicus:

    • The most common isolate from respiratory system diseases
    • May result in a venereal infection – which could cause sporadic abortions
    • Could be responsible for paralysis in foals (polyarthritis)
  • Strangles is can be diagnosed on clinical signs, although not all horses develop the characteristic swollen lymph nodes of the head and neck. Historically, nasal washes and swabs have been used to culture the bacteria. However, this method of diagnostics may not always be accurate, as the horse may not be shedding bacteria at an early stage of infection, or the bacteria may not survive transportation. The most reliable method is involves DNA testing of the bacteria, which also allows differentiation of the two subspecies, S. equi and S. zooepidemicus.

  • Treatment of the infected horse:

    • Appropriate treatment of horses with strangles usually depends on the stage and severity of the disease. Veterinary opinion as to whether or not to use antibiotic treatment remains markedly divided. However, the majority of strangles cases require no treatment other than proper rest and a dry, warm stall and provision of soft, moist, and palatable food of good quality while letting the disease run its course. Food and water should be easily accessible to the horse.

    • Horses with Early Clinical Signs
      During an outbreak, immediate antibiotic therapy of new cases in the early acute phase with fever and depression may be curative and may prevent focal abscessation. Penicillin is generally considered the drug of choice for the treatment of nonpneumococcal streptococcal disease, with alternative drugs used depending on ease of administration or the site of infection.

  • Good hygiene and management is essential to stop the spread of S. equi infections.

    Infected and clinically normal horses should be segregated. Precautions should be taken, and may include:

    • Exercise of infected horses scheduled after general population’s exercise period to avoid potential bacterial transfer to unaffected horses/barns by exercise riders
    • No access by infected horses to starting gate or similar equipment
    • Direct horse-to-horse contact is to be avoided
    • Prompt post-contact use of hand sanitizer by individuals having contact with horses
    • during exercise

    Vaccination has been shown to greatly reduce the incidence of strangles, and the severity of the related symptoms. Vaccination is indicated for use in healthy horses only (ie those not showing clinical symptoms of strangles, or any other disease), and, where available, is recommended as an essential tool in preventing a strangles outbreak.